1. Field of the Invention
This invention relates to apparatus for testing MOS-LSI devices and, more particularly, to measuring the strength of bonds which join the beam leads and the device chips.
2. Description of the Art
There exists a need for test equipment that can accurately test and measure the bond strength and observe the mode of failure of individual beam leads. To ascertain the reliability of the fabricated beam leads at least two factors must be measured, the beam strength and the rate of aging. The bond pull strength is critical in determining both of these factors hence a test apparatus that can measure both the force necessary to break a bond and observe the mode of failure is desirable. Integrated circuit chips are generally very small, fragile and involve a plurality of beam leads that are bonded to the chips. Presently, bond test equipment lacks the capability of gripping each individual minute beam lead and stressing it under varied geometrical conditions until failure occurs. That is, since the beam leads are small and intricate to handle, it is difficult to position the test equipment in relation to the chips and the beam leads to hold one end and pull the beam lead until it fails and simultaneously measure the force required for such failure. Adams et al, in Review of Scientific Instruments, November, 1968, p. 1768, teaches how to grip fine wires by using a small drop of apiezon wax on the end of a thin probe, then manipulating the probe mechanically to make contact with the wire to be tested. A heating element is brought into contact with the probe and current is turned on causing the wax to melt, capillary attraction causes the wax to surround and "grab" the wire, thus producing a rigid and strong bond. While this means appears to be suitable for gripping fine wires, it subjects the wire to heat from a heating element with electric current and also requires the use of wax. Both of these factors are time consuming and could distort the true force required to test a fine wire.
Another known apparatus for testing the bond strength between two laminated layers of material is taught by A. R. Mancini in U.S. Pat. No. 3,019,644. Mancini discloses the use of an electric motor and a force gage to measure the stripping force of two laminated layers. The apparatus consists of a rigid frame having a common drive shaft connected to several structural elements including sprocket gears, sprocket chain, drive pulleys, idler pulleys, a bifurcated support member, hook and a spring scale. Both layers are slowly advanced by the mechanization in opposite directions until there is a failure, the spring scale measures the force. This type of apparatus is too cumbersome and complex to measure the bond strength of minute beam leads. Also, gripping the beam leads is difficult with a hook.
U.S. Pat. No. 2,377,869 by M. A. Elliott also teaches the use of an electric motor and drive members to test the deep drying properties of insulating materials. This apparatus includes various electrical components that make up a measuring part of the apparatus. The specimen to be tested is held between two clamps. One clamp is suspended from a tension bar. The first clamp is drawn away from the second clamp by a drive motor. The meausring part of the apparatus measures the voltage change with respect to the deflection of the tension bar which is proportional to the actual tension to which the specimen is subjected. This apparatus is also quite complex, and it is doubtful that it could be adapted to test beam leads.
McGrath, U.S. Pat. No. 3,702,437 discloses a micrometer driver positioning assembly for moving and supporting an electronic assembly during diagnosing and repairing of devices of said electronic assembly. This apparatus lacks a means for clamping the small wires and the micrometer driving assembly does not slide around independently of the micrometer which is used for fine positioning. ;p Still another apparatus, the "Hunter Terminal Pull Tester" described in Bulletin 750e of Hunter Spring Company, dated September, 1961, tests electrical connectors. This apparatus utilizes an air motor having a moving piston which connects directly to a set of serrated clamping jaws for gripping the wire. An indexing disc provides holding notches for terminals of varying sizes which is mounted to a force gage. Once the crimped terminal or connector is secured in the notch on the tension head of the force gate, and the wire is placed between the open jaws of the air motor, the jaws close around the wire and the piston moves at the preset speed. The piston pulls the jaws and wire until the crimped connection fails or the insulation breaks. The force gage indicates the force exerted to cause the instant failure. While the Hunter tester appears to be adequate for production type terminals, more precise equipment and means is required for securing the minute integrated chips, clamping of the fine beam leads, measuring the beam lead strength and determining the mode of failure.